1. Technical Field of the Invention
The present invention relates to a semiconductor device equipped with MOS transistors (Metal Oxide Semiconductors) having different drain breakdown voltages on the same semiconductor layer, and a method for manufacturing the same.
2. Background Technology
Currently, field effect transistors with LOCOS (Local Oxidation Of Silicon) offset structure are known as field effect transistors that are designed to have a higher breakdown voltage. A field effect transistor having a LOCOS offset structure is a transistor that is provided with a LOCOS layer between a gate dielectric layer and a drain region, and an offset impurity layer below the LOCOS layer.
Also, along with the trend of further weight-reduction and miniaturization of various electronic devices in recent years, there are demands in further reduction in the size of ICs to be mounted on these electronic devices. In particular, for driving ICs in electronic devices having liquid crystal display devices, technologies that reduce chip areas of the ICs through mix-mounting low voltage driving transistors for low voltage operation and high breakdown voltage transistors for high voltage operations on the same substrate (the same chip) are in strong demand. When a high breakdown voltage transistor provided with a LOCOS layer for alleviation of the electric fields described above and a low voltage driving transistor are formed on the same substrate, a semiconductor device of such a structure as described above can be manufactured through, for example, forming a LOCOS layer for element isolation and a LOCOS layer for alleviation of the electric fields in the same step.
However, due to the demands in further miniaturization in recent years, methods for forming element isolation regions are shifting from LOCOS methods to STI (Shallow Trench Isolation) methods, and a method in which LOCOS layers for alleviation of the electric fields of high breakdown voltage transistors are replaced with trench dielectric layers has been proposed.
It is an object of the present invention to provide a semiconductor device in which high breakdown voltage transistors and low voltage driving transistors are formed on the same substrate, and a semiconductor device and a method for manufacturing the same wherein LOCOS layers and trench dielectric layers are commonly used, such that further miniaturization and improved reliability can be achieved.